Solution: Missing product key for Windows 8 Media Center Pack

So you’ve just installed Windows 8 Pro and you’ve entered your e-mail adress in order to get the free Media Center with it. But it haven’t arrived yet.

According to this source you’ll have to give it between 48 to 72 hours in order to arrive in your inbox.
Also you should keep in mind that if it haven’t arrived within 72 hours it could have ended up in your spam-folder, so check your spam folder, once in a while – just in case.